1. The Field of the Invention
The present invention relates generally to optical transmitters. More specifically, the present invention relates to an interconnect mechanism for connecting a laser driver to a laser with high signal fidelity and low voltage and power loss.
2. Background and Relevant Art
Computing and networking technology have transformed our world. As the amount of information communicated over networks has increased, high speed transmission has become ever more critical. Many high speed data transmission networks rely on optical transceivers and similar devices for facilitating transmission and reception of digital data embodied in the form of optical signals over optical fibers. Optical networks are thus found in a wide variety of high speed applications ranging from as modest as a small Local Area Network (LAN) to as grandiose as the backbone of the Internet.
Typically, data transmission in such networks is implemented by way of an optical transmitter (also referred to as an electro-optic transducer), such as a laser or Light Emitting Diode (LED). The electro-optic transducer emits light when current is passed through it, the intensity of the emitted light being a function of the current magnitude. An electro-optic transducer driver generates the appropriate magnitude of current to pass through the electro-optic transducer to generate the appropriate amount of optical intensities corresponding to the data being transmitted.
In order to assert one binary value, a relatively low current (called herein “IBIAS”) is passed through the electro-optic transducer to thereby cause a relatively low optical power level to be transmitted onto the optical fiber. In order to assert the opposite binary value, a relatively high current is passed through the electro-optic transducer to thereby cause a relatively high optical power level (e.g., IBIAS plus a maximum modulation current called herein “IMOD”) to be transmitted onto the optical fiber. Accordingly, by superimposing a modulation current (that varies between zero and IMOD) upon the bias current, an appropriate sequence of bits may be transmitted.
FIG. 1 illustrates a driver-transducer circuit 100 that includes an electro-optic transducer 101 in the form of a specially manufactured diode. Methods for fabricating electro-optic transducer 101 in the form of a diode are well known in the art. The optical power transmitted by the electro-optic transducer 101 is approximately proportional to the amount of current passed through the electro-optic transducer 101 for the frequency range of interest.
FIG. 1 also illustrates an electro-optic transducer driver output stage 110. The electro-optic transducer driver output stage 110 applies the appropriate current through the electro-optic transducer 101 depending on the data. In the illustrated embodiment, the electro-optic transducer 110 is what is referred to as “DC-coupled” to the electro-optic transducer 101. Note that although it is not illustrated in FIG. 1, transmission structures such as transmission lines may be utilized between electro-optic transducer driver output stage 110 and transducer 101.
Specifically, a bias current source 111 draws a bias current IBIAS through the electro-optic transducer 101. In addition, a modulation current source 112 draws the maximum modulation current IMOD through either the bipolar transistor 121, or the bipolar transistor 122, or through both of the bipolar transistors 121 and 122 in a split manner. The amount of modulation current IMOD drawn through the electro-optical transducer 101 depends on the differential data signals DATA and DATA! applied at the base terminal of the corresponding bipolar transistors 121 and 122.
The DC-coupled circuitry 100 of FIG. 1 is advantageous in that the modulation current is drawn completely through the electro-optic transducer 101 through the bi-polar transistor 122. In addition, bias current IBIAS is used to bias both electro-optical transducer 101 and driver output stage 110. Accordingly, the circuit 100 is highly efficient.
However, DC-coupling is generally a single-ended strategy to drive electro-optical transducer 101. In other words, electro-optical transducer 101 must be connected to either the output of transistor 121 or transistor 122, but not both. For example, electro-optic transducer 101 is connected to transistor 122 while transistor 121 is terminated by a termination resistor 105 that is used to balance the complimentary transistor pair. Accordingly, DC-coupled circuitry 100 does not take full advantage of the symmetry created by complimentary nature of transistors 121 and 122. For instance some the speed and signal fidelity advantages created by the complimentary pair 121 and 122 may be lost in the single-ended configuration.
Furthermore, as the DC-coupled circuit 200 is single-ended, it requires a very low inductance ground in the current return path. This is because all the current is returned to ground via only one of the transistors, instead of both transistors as in a differential circuit. For example, the current return path to the ground is at bias source 111. Low impedance grounds, however, are difficult to manufacture in optical transmit circuits.
FIG. 2 illustrates another conventional driver-transducer circuit 200. In this circuit, the modulation current of the electro-optic transducer driver output stage 210 is “AC-coupled” to the electro-optic transducer 201. A bias current source 211 supplies the bias current IBIAS plus IMOD/2 through the electro-optic transducer 201. A modulation current source 212 causes modulation current to pass through the electro-optic transducer 201 through AC-coupling capacitors 225A and 225B.
Specifically, the modulation current source draws 1/n times (where “n” is the AC coupling coefficient) the maximal modulation current IMOD in a split manner through the bipolar transistors 221 and 222. The amount of current drawn through pull-up resistor 223 and through bipolar transistor 221 depends on data signal DATA and DATA!. Accordingly, the amount of current drawn through source pull-up resistor 224 and bipolar transistor 222 depends on data signal DATA and DATA! as well, since the sum of current drawn through both bipolar transistors 221 and 222 remains constant at IMOD/n.
The amount of current drawn through bipolar transistor 222 may thus vary from zero to IMOD/n, depending on the data signal DATA. Conversely, the current drawn through bipolar transistor 221 may vary from zero to IMOD/n as well, in a complementary manner to the current drawn through bipolar transistor 222. The resulting differential current is AC-coupled through transmission mechanism 205. For example, differential current is AC-coupled through the capacitors 225A and 225B, through corresponding transmission lines 226A and 227B, and through corresponding load resistors 227A and 227B (each having resistance RL) so that only the fraction equal to the AC-coupling coefficient “n” of the differential current passing through bipolar transistors 222 and 221 is provided through the electro-optic transducer 201. Therefore, the modulation current provided through the electro-optic transducer 301 varies from zero to IMOD, depending on the data signal DATA.
The AC-coupled driver-transducer circuit 200 of FIG. 2 is advantageous in that the circuit takes full advantage of the complimentary nature of transistors 221 and 222 in terms of speed and signal fidelity. However, the AC-coupled driver-transducer circuit 200 does have a significant disadvantage. The driver circuit output stage 210 in the AC-coupled driver-transducer circuit 200 must draw more modulation current than the driver circuit output stage 110 in the DC-coupled driver-transducer circuit 1100 of FIG. 1. For instance, the modulation current drawn by driver circuit 210 is IMOD/n, where “n” (the coupling coefficient) is less than one, and is ideally around 50% for optimal performance. In addition, the coupling capacitors 225A and 225B make it so driver output stage 210 and transducer 201 must use separate biasing currents, thus using power less efficiently.
As an additional disadvantage, the presence of the load resistors 223 and 224 means that the driver circuit 210 must generally operate using higher supply voltages as the load resistors may cause a large voltage drop. The driver circuit 210 of the DC-coupled configuration may operate at 3.3 volts, whereas the driver circuit 310 of the AC-coupled configuration may use supply voltages of 5 volts.